1. Field of the Invention
The invention relates to video signal compression and more particularly to video signal compression apparatus and methods which repeatedly sample image information for a set of basic colors to produce a sequential color video signal.
2. Description Relative to the Prior Art
In developing a video signal recording or transmission system, bandwidth compression of the image representative signals becomes a basic practical concern. Such compression must be accomplished, however, while preserving sufficient image detail to provide a final display image of satisfactory quality. Considering the standard NTSC signal format, for example, concern for bandwidth conservation is evidenced in use of special chrominance interleaving and in the selection of bandwidth allocations among the three signal types: Y, I, and Q. These particular techniques are effective to compress overall signal bandwidth because the NTSC video signal is a composite of analog signals which occur concurrently and bear dissimilar quantities of information.
Another signal format which has been proposed intermixes basic color samples in a repeated sequence (e.g., green, then red, then blue) along a scan line to produce a sequential output signal. This signal format is generally referred to as "dot-sequential" and results in a time division multiplexed color video signal (see "A Six-Megacycle Compatible High-Definition Color Television System" by RCA Laboratories Division, Princeton, New Jersey, published in RCA Review, December 1949, pages 504 through 522). This approach relies on integration of the "dot" elements along the scan lines of a final display by the human visual system to "fill in the gaps" between dots and, in doing so, seeks to save signal bandwidth by updating each basic type of image information only intermittently. To provide, with the dot-sequential signal format, greater information concerning luminance detail, a relatively high frequency signal is typically superposed on the sequential signal. (See RCA Laboratories Division article mentioned above.)
National standardization on the NTSC format precludes use of the "dot-sequential" format for broadcasting; nonetheless, significant use possibilities exist in the fields of video recording and in closed circuit video transmission. One difficulty with using the dot-sequential format, however, arises as a result of the large number of elements (dots) which are necessary to represent an image of satisfactory quality. Moreover, while this type of format is, because of its sequential nature, well suited to digital encoding, such encoding is not readily compatible with the use of a concurrent luminance signal unless a separate channel is used.